SCMA is a non-orthogonal multiple access scheme that allows multiple devices, users, or user equipments (UEs) to share channel resources. Potential transmit devices are allocated time and frequency resources, also referred to as resource units. In SCMA, potential transmit devices are also assigned a sparse codebook that allows superposition of device transmissions, which allows SCMA systems to support more connected devices.
In grant-free SCMA systems, there is no signaling from the network to devices for scheduling transmissions. The devices access the shared channel resources in a contention based manner. Contention based access begins to break down when two or more devices attempt to transmit using the same resources, which is referred to as a collision. SCMA can tolerate some amount of signal collision. SCMA systems can control the probability of collision using collision avoidance and collision detection and recovery techniques to mitigate its impact. For additional information regarding SCMA, refer to U.S. application Ser. No. 13/730,355, which is hereby incorporated herein by reference.
Base station receivers generally need arriving signals to be synchronized in time in order to receive them correctly and take advantage of low complexity message passing algorithm (MPA) receivers. That synchronizing is typically achieved through signaling from the base station to the transmitting devices indicating timing adjustments to transmit timing.